1. The Field Of The Invention
The present invention relates to an improved conductor pattern for receiving integrated circuit chips directly attached thereon and in particular to an improved configuration of a metallized pattern of leads on a flexible insulator substrate which supports the individual leads throughout their entire length and yet provides stress relief for the connection points between the pattern and devices attached thereto.
2. The Prior Art
A well known metal lead frame device and its method of manufacture are described in U.S. Pat. No. 3,440,027. This lead frame has fully supported conductors disposed on a flexible insulator substrate. U.S. Pat. Nos. 3,689,991 and 3,763,404 both show another type of lead frame which is commonly known as a beam-over-hole lead frame. According to these patents, at least a portion of each conductor extends in cantilever fashion over a hole in the flexible substrate. Both the fully supported and the beam-over-hole types of lead frames have a number of disadvantages. For example, the fully supported type has the particular disadvantage that undue stresses can build up on the leads themselves due to differential thermal expansion between the metallic leads and the insulative substrate when heat is applied to bond a chip thereto and/or by thermal variations during normal use. The heat can cause sufficiently large distortion of the substrate during the bonding of the leads to the chip to cause the bonded joint to be defective or to even break the chip during later thermal cycling, should the bonds be of sufficient strength. This is especially true with large multiple bonded structures such as MSI and LSI. The beams-over-hole arrangement described in the other two patents does not have the above-discussed problem of undue stress buildup but instead has a problem of supporting the beams, during fabrication and handling, in a coplanar predetermined array prior to the bonding of chips thereto. There is a further disadvantage in this arrangement in that the leads can be easily and permanently damaged prior to their use, thus substantially increasing waste and cost.
Apparatus of a type suitable for attached lead frames of the above-described types are shown in U.S. Pat. Nos. 3,317,287, and 3,724,068.
The conventional method of attaching integrated circuit chips to a circuit has been a two step process. First the chip is bonded to a lead frame of one of the known types. This subpackage is then separated from a carrier and bonded to the final circuit. Clearly this method has the disadvantage of multiple handling steps during which the leads can easily be misaligned making the package useless.
There are many well known additive and subtractive methods for forming electrical circuitry of the subject lead type on either flexible or rigid insulating substrates. The primary disadvantage of the known additive methods is that they are relatively slow in that they all require an electroless deposition step to plate the metal layer on the substrate. These known additive methods generally are similar in that they comprise the steps of coating a non-conductive flexible or rigid substrate with a photo resist material, exposing a circuit pattern to the photo resist, and stripping away the photo resist to leave a circuit pattern that can be plated. The prepared substrate, after catalyzing the pattern by known chemical processes, is then placed in an electroless plating tank and allowed to stand until a sufficient buildup of metal occurs on the treated portions of the substrate. While the known additive methods have proved to be quite satisfactory in forming electrical circuits, they are all relatively slow as compared to electroplating and using present state of the art solution technology. They also suffer from reduced adhesion between the plating and the substrate.
In the known semi-additive methods, a substrate is completely blanket metallized to a sufficient thickness to be conductive. A photo resist is applied, exposed and developed and the circuitry is electroformed and covered with an etch resistant metal as the top layer. The photoresist is then removed and the initial blanket metallization etched away leaving only the desired circuitry.
Another known method for forming electrical circuitry on flexible or rigid insulating substrates is the subtractive method. This method generally comprises printing a circuit pattern on a metal sheet and etching the circuit out of the sheet. The metal sheet can be laminated to an insulator substrate either before or after the etching step. There is the problem of the type of adhesive to use in order to prevent chemical attack by the etchant as well as smearing of the adhesive when a formed circuit is joined to a substrate after etching an intermediate substrate. There is also the problem of undercutting the fine conductor circuitry thus causing poor resolution of the etched pattern.
Thus the additive and subtractive techniques for forming a metallized circuit on an insulative substrate can be summarized as follows: the additive method starts with an uncoated substrate and is masked in all areas where circuitry is not desired, while the subtractive method masks those areas of a coated substrate where the circuitry is desired.